Where is "Regular Solids" option?
The plugin you need is called now add_mesh_extra_objects.
To enable it press CtrlAltU to open the preferences window.
On the Addons tab type extra and and check the box for Add Mesh:Extra Objects



There are new options on the add mesh menu. Select Math Function Solid.



On the tools tab (T) select Icosahedron



Alternatively press F6 to access the option for the tool.